A. Field of the Invention
The present invention relates to a compact zoom lens system having a high zoom ratio.
B. Description of the Prior Art
Recently, so called "compact" cameras tend to have zoom features in addition to having automatic features and being compact. Historically, zoom lenses have been widely used for single lens reflex cameras, and various types of zoom lenses for such applications have been proposed. However, zoom lenses designed for single-lens reflex cameras are not suitable for compact camera applications because such zoom lenses have a long back focus distance and are not sufficiently compact.
For compact cameras, two types of zoom lenses have been proposed. The first type is composed of two lens groups and typically has an approximately 1.5 times zoom ratio. However, for a zoom ratio greater than 2.0, this first type can not practically be made compact.
The second type is composed of three lens groups and typically has an approximately 2.5 times zoom ratio. U.S. Pat. Nos. 4,978,204; 5,002,373; and 5,033,832 have proposed the second type. However, the zoom ratio of this second type is limited to an approximately 2.6 times, and this zoom ratio is not sufficient for compact camera applications.
For example, U.S. Pat. Nos. 4,978,204 and 5,002,373 proposed zoom lenses of the second type having an approximately 2.6 times zoom ratio and composed of eleven elements. U.S. Pat. No. 5,033,832 proposed zoom lenses of the second type having an approximately 2.7 times zoom ratio and composed of twelve (12) elements. The zoom lenses proposed in the '832 patent widen the viewing angle at an wide angle position, but its 2.7 times zoom ratio is still insufficient. For compact cameras, the zoom ratio of at least 3.5 times is desired.